Download Wksht. Calculating Average Atomic Mass I

Mr. Scharff
Intro to Chemistry
Name ____________________
Date ______________ Pd. ___
State of Ohio GLI
 PS 9.1. Recognize that all atoms of the same element contain the same number of protons, and elements with the same number of protons
may or may not have the same mass. Those with different masses (different numbers of neutrons) are called isotopes.
Wksht. Calculating Average Atomic Mass I
Elements come in a variety of isotopes, meaning they are made up of atoms with the same atomic number but different
atomic masses. These atoms differ in the number of neutrons. Following are the examples illustrating the written name of a
specific atom or isotope: C-14 or 14C for carbon-14
The average atomic mass is the weighted average of all the isotopes of an element (see below).
Example: A sample of cesium is 75% Cs-133, 20% Cs-132, and 5% Cs-134.
What is its average atomic mass?
Answer:
0.75 x 133 = 99.75
0.20 x 132 = 26.4
0.05 x 134 = 6.7
132.85 amu’s (average atomic mass)
Directions. Define the following terms. Determine the average atomic mass for the following mixtures of isotopes. You
must show all mathematical calculations to receive credit.
Atomic mass (mass number) = _________________________________________________________________________
Atomic number = ____________________________________________________________________________________
A. 80% I-127, 17% I-126, 3% I-128.
B. 50% Au-197, 50% Au-198
Distasio, J. Chemistry Grades 9-12, Reproducible Activities. McGraw-Hill Children’s Publishing. ISBN 1-568822-187-8. Pg. 28. Edited K.Scharff, 11/15/2010.